The present invention generally relates to rotary machines, such as steam and gas turbines, and, more particularly, is concerned with a rotary machine having a seal assembly for controlling clearance between tips of rotating rotor blades and a stationary outer casing of the rotary machine.
Steam and gas turbines are used, among other purposes, to power electric generators, and gas turbines also are used, among other purposes, to propel aircraft and ships. A steam turbine has a steam path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Compressor and turbine sections include at least one circumferential row of rotating blades. The free ends or tips of the rotating blades are surrounded by a stator casing.
The efficiency of the turbine depends in part on the radial clearance or gap between the rotor blade tips and the surrounding casing. If the clearance is too large, more of the steam or gas flow will leak through the gap between the rotor blade tips and the surrounding casing, decreasing the turbine""s efficiency. If the clearance is too small, the rotor blade tips may strike the surrounding casing during certain turbine operating conditions.
It is known that the clearance changes during periods of acceleration or deceleration due to changing centrifugal force on the blade tips and due to relative thermal growth between the rotating rotor and stationary casing. During periods of differential centrifugal and thermal growth of the rotor and casing the clearance changes can result in severe rubbing of the moving blade tips against the stationary casing. This increase in blade tip clearance results in efficiency loss.
Clearance control devices, such as rigid abradable shrouds, have been proposed in the past to accommodate rotor-to-casing clearance change. However, none are believed to represent an optimum design for controlling such clearance. Consequently, a need still remains for an innovation which will provide a solution to the above-described clearance control problem without introducing any new problems in place thereof.
The present invention provides a rotary machine with a flexible abradable seal assembly designed to satisfy the aforementioned need. The flexible abradable seal assembly controls the radial clearance between the rotating rotor blades and the stationary outer casing of the rotary machine by flexing and abrading in response to contact therewith by the blade tips due to differential growth of the rotor and blades relative to the casing and thereby reduces contact severity by moving abradable layer with blade. Compared to commonly used rigid abradable shrouds, the flexibility of the seal assembly of the present invention will result in less heat generation and material removal. The reduced wear rates of the seal assembly will provide tighter blade tip clearances for longer periods yielding better operating efficiency.
In an embodiment of the present invention, a rotary machine is provided which includes a rotor rotatable about a longitudinal axis and having an outer surface, a plurality of blades mounted on and spaced from one another circumferentially about and extending radially outward relative to the longitudinal axis and from the outer surface of the rotor to end tips of the blades, an outer casing having an annular shape and an inner circumferential surface and being stationarily disposed about and spaced radially outwardly from the rotor and the blades so as to define an annular gap between the inner circumferential surface of the outer casing and the end tips of the blades, and a flexible abradable seal assembly attached on the inner circumferential surface of the outer casing and disposed within the annular clearance gap between the casing and blade tips such that, during periods of differential growth of the rotor and blades relative to the casing, the seal assembly flexes and abrades in response to contact therewith by the tips of the blades moving with the rotor and thereby clearance between the casing and rotor blades at the annular gap being controlled in such manner as to avoid contact by the moving blade tips with the casing.
Different embodiments of the flexible abradable seal assembly are disclosed. In each embodiment the seal assembly basically includes an elongated flexible body having an arcuate shape generally conforming to the annular shape of the inner circumferential surface of the outer casing, a pair of opposite spaced apart longitudinally extending edge portions spaced respectively upstream and downstream of the end tips of the rotor blades, and a main body portion extending between and interconnecting the opposite edge portions and being disposed outwardly of and aligned with the end tips of the rotor blades. The elongated flexible body has a portion disposed adjacent to the end tips of the rotor blades that is a material abradable in response to contact therewith by the end tips of the rotor blades. In the illustrations, the flexible body is made up of an annular array of a plurality of like arcuate-shaped body segments disposed end-to-end with one another.
In each embodiment the seal assembly further basically includes means for securing the flexible body at the opposite edge portions thereof to the inner circumferential surface of the outer casing. As examples, the securing means can be welds or fasteners.